Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Although we see a lot of ugliness in the characters, it is spread around fairly evenly. Morrison shows the good and bad in nearly all the major players, and she often follows up the most shocking actions with an explanation of the character's motivation. Take Sula, for example. After she sleeps with Jude, and when we're primed to hate her, Morrison offers us this passage:
Marriage, apparently, had changed all that, but having had no intimate knowledge of marriage, having lived in a house with women who thought all men available, and selected from them with a care only for their tastes, she was ill prepared for the possessiveness of the one person she felt close to. She knew well enough what other women said and felt, or said they felt. But she and Nel had always seen through them. They both knew that those women were not jealous of other women; that they were only afraid of losing their jobs. After their husbands would discover that no uniqueness lay between their legs. (1939.39)
Morrison doesn't make excuses for Sula here, but she does offer us a possible explanation for why she doesn't understand the betrayal Nel feels. So while we might still condemn Sula's actions, Morrison does provide an explanation for them. She doesn't judge acts of adultery, murder, or accidental drowning. She presents them to us and lets us reach our own conclusions.